1. Technical Field
The present invention relates to a grazing incidence interferometer.
2. Related Art
Various interferometers for measuring a surface texture of a workpiece are known, and normal incidence interferometers are commonly used for this purpose. Whereas normal incidence interferometers can perform high accuracy measurement using the wavelength of light as a reference, they cannot measure a texture of a measurement subject having a discontinuous step whose height is larger than half of the wavelength or a large undulation that produces an image having a height variation that is larger than half of the wavelength between adjoining pixels.
Grazing incidence interferometers are known as interferometers capable of measuring large undulations that cannot be measured by normal incidence interferometers (refer to Patent document 1).
In grazing incidence interferometers, the apparent wavelength can be increased by applying light obliquely and using resulting reflection light, whereby wavefront variations with respect to an undulation of a measurement subject can be reduced intentionally. Furthermore, since reflection light directions are equalized because of grazing incidence of measurement light, clear interference fringes as would be obtained with a glossy surface can be obtained even with a rough surface.
In grazing incidence interferometers, in general, the distance representing an optical path difference of one wavelength is called fringe sensitivity and defined as a height difference per interference fringe Λ=λ2 cos θ (μm) where λ is the wavelength of measurement light and θ is the incident angle.
The fringe sensitivity is determined by the incident angle of measurement light and the wavelength of a laser (light source). For example, where the laser wavelength is fixed, the fringe sensitivity is determined only by the incident angle. Therefore, the incident angle is set taking surface properties and condition of a measurement subject and required measurement accuracy into consideration.
Surfaces having such a relatively large undulation that measurement by normal incidence interferometers is difficult (mentioned above) and rough surfaces (non-specular surfaces) are assumed as measurement subjects of grazing incidence interferometers. Typical measurement subjects are various wafers and glasses for FPDs (flat panel displays).
In FPD glasses etc., to attain high accuracy, flatness management which is performed before polishing is important. In this respect, high-accuracy flatness measurement using a grazing incidence interferometer is counted on very much. In recent years, the sizes of FPDs have increased because of their application to large-screen TV receivers and hence the needs of flatness management of large, high-accuracy non-specular surfaces as measurement subjects have been increasing.
To widen the measurement ranges of grazing incidence interferometers to satisfy the requirement of size increase of measurement subjects, the following measures are now being studied.
First, the measurement area of a grazing incidence interferometer can be widened by increasing the incident angle of measurement light.
If the incident angle of a grazing incidence interferometer is increased, the illumination area of a measurement subject surface becomes an elliptical area that is longer than the diameters of a laser light beam in the direction corresponding to the incident direction and the measurement area is widened accordingly. However, the measurement resolution lowers as the measurement area is widened. Therefore, this method may be unfavorable for required measurement accuracy.
Second, a texture of the entire measurement surface of a measurement subject can be measured through calculation by dividing the measurement surface into plural sections, measuring the individual sections successively while moving an interferometer, and connecting measurement results of the respective sections together after completion of the measurement operation.
Such a scanning measurement method was proposed for a normal incidence interferometer (refer to Patent document 2). In the method disclosed in this document, a specular surface of a measurement subject is measured by a normal incidence interferometer and the posture of the normal incidence interferometer is recognized correctly using another interferometer, whereby high accuracy of the connections between measurement results of respective sections is secured.
Applying this scanning measurement method to grazing incidence interferometer enables wide-range measurement while maintaining the height resolution of the grazing incidence interferometer.
[Patent document 1] JP-A-2010-32342
[Patent document 2] JP-A-2000-88551
The above-described method of increasing the incident angle of measurement light of a grazing incidence interferometer to widen its measurement range is not suitable for the increasing of accuracy because of reduction in resolution. In addition, the measurement range increase that is attained by increasing the incident angle is restricted.
On the other hand, the scanning measurement method using a grazing incidence interferometer can widen the measurement range to a large extent by connecting measurement results of respective measurement sections together.
However, to perform such a scanning measurement, it is necessary to support a grazing incidence interferometer for measuring the surface of a measurement subject on a moving mechanism and to prepare another interferometer for measuring the posture of the grazing incidence interferometer. An interferometer needs to be provided with a series of devices such as a laser oscillator, optical paths, and photodetectors. The use of another interferometer causes doubling of these devices, which necessarily complicates the configuration, maintenance work, etc., increases the cost, and causes other problems.